When using industrial robots for carrying out various work operations on objects moving along a production line, there is often a need to shorten the operating times at the work stations. This can be done, in principle, by extending the number of robots at the stations in question. However, the existing floor area sets limits to the number of robots that can be accomodated. This has resulted in solutions by which the robot is mounted on an overhead stand which extends across the assembly line. Owing to the weight of the robot, such a stand must be made very strong and stable, since the robot will otherwise cause movements and deformations of the stand with a resultant deteriorated precision as regards the operation of the tool mounted on the tool attachment of the robot. If more robots are mounted on the same stand, this problem will become more evident.
Modern industrial robots normally operate with rapid movements which are repeated very frequently. Such robots are often used for spot welding, and in that connection it is sometimes a requirement from an economic point of view that they shall be capable of accelerating and decelerating within a time of less than 1 second. If the mechanical system of the robot is in unbalance during these movements, vibrations will arise resulting in deteriorated manufacturing quality. In addition, such a mechanical unbalance requires a higher power output, and therefore the drive motors have to be dimensioned correspondingly stronger.
From, for example, U.S. Pat. No. 4,507,043 it is known, in an industrial robot, to counterbalance the secondary arm of the robot by means of a counterweight. However, the described robot cannot--as a pendulum robot--be suspension-mounted upside-down on an overhead stand structure arranged across a production line. Further, this robot is not suitable for work in places where space is limited, for example for spot welding inside car bodies, etc.
From U.S. Pat. No. 3,750,895 it is further known to arrange, in connection with tower cranes with a horizontal beam, a secondary beam which is linearly displaceable on the first-mentioned beam, said secondary beam being connected by means of wires to a movable counterweight arranged on the opposite side of the crane mast. The task of the counterweight is to reduce the bending moment on the crane mast. In this case, contrary to the case of an industrial robot, it is not a question of a dynamic counterbalancing of an arm system which is rotatable about two axes crossing each other.